1. Field of the Invention
The present invention relates to a longitudinally coupled resonator type balanced surface acoustic wave filter having a balanced-unbalanced conversion function, and in particular, to a balanced surface acoustic wave filter including five IDTs arranged along a surface acoustic wave propagation direction.
2. Description of the Related Art
In recent years, balanced surface acoustic wave filters having a balanced-unbalanced conversion function have been used for bandpass filters in an RF stage of cell phones. In particular, longitudinally coupled resonator type balanced surface acoustic wave filters have been primarily used since the longitudinally coupled resonator type balanced surface acoustic wave filters can support high frequencies and can easily provide a balanced-unbalanced conversion function.
A balanced surface acoustic wave filter having a balanced-unbalanced conversion function is connected to a balanced mixer IC having a balanced input/output or a differential input/output. By using this balanced mixer IC, the balanced surface acoustic wave filter can reduce the effect of noise and stabilize the output thereof. Accordingly, in recent years, balanced mixer ICs have been widely used in order to improve the characteristics of cell phones. Surface acoustic wave filters connected to a balanced mixer IC must have a balanced-unbalanced conversion function.
A variety of types of surface acoustic wave filter having a balanced-unbalanced conversion function have been disclosed. An example of such a balanced surface acoustic wave filter is described in Japanese Unexamined Patent Application Publication No. 2004-96244 (Patent Document 1).
FIG. 7 is a cross-sectional plan view schematically illustrating the balanced surface acoustic wave filter described in Patent Document 1.
A balanced surface acoustic wave filter 101 includes a one-port surface acoustic wave resonator 103 to be connected to an unbalanced terminal 102. A surface acoustic wave filter unit 104 is connected downstream of the one-port surface acoustic wave resonator 103. The surface acoustic wave filter unit 104 includes first to fifth IDTs 105 to 109 disposed in that order along the surface acoustic wave propagation direction. Reflectors 110 and 111 are disposed at either end of an area in which the IDTs 105 to 109 are disposed in the surface acoustic wave propagation direction.
Here, the second IDT 106 and the fourth IDT 108 are connected to the unbalanced terminal 102 via the one-port surface acoustic wave resonator 103. The second IDT 106 and the fourth IDT 108 have a phase difference of 180° with respect to each other. The third IDT 107 is divided into a first sub-IDT portion 107a and a second sub-IDT portion 107b in the surface acoustic wave propagation direction.
The first IDT 105 and the first sub-IDT portion 107a are mutually connected and are connected to a first balanced terminal 112. Additionally, the second sub-IDT portion 107b and the fifth IDT 109 are mutually connected and are connected to a second balanced terminal 113. In recent years, for a surface acoustic wave filter having a balanced-unbalanced conversion function, a low insertion loss in the passband and a terminal impedance that can be easily adjusted have been highly desirable. In addition, compact bodies of surface acoustic wave devices have been highly desirable.
Similar to the balanced surface acoustic wave filter 101, longitudinally coupled resonator type balanced surface acoustic wave filters including five IDTs 105 to 109 advantageously achieve the above-described requirement.
However, although the surface acoustic wave filters including the 5-IDT balanced-unbalanced conversion function, such as the balanced surface acoustic wave filter 101, can provide a reduction of the insertion loss in the passband, a terminal impedance capable of being easily adjusted, and a reduction in the body size, undesired spurious components are generated in the vicinity of the passband.